In recent years many proposals have been offered to protect humans and property which inadvertently come into contact with a current conducting line of an electrical supply system, or which unexpectedly are subject to high voltage and thus shock and damage because of some fault occurring in the supply system. Devices known as Ground Fault Current Interrupters (GFCI) are known for use in electrical supply systems as means for attempting to provide the desired safety to humans, machinery and other property. Many of these devices employ a differential transformer whose primary windings are the current carrying lines of the supply system and whose secondary winding is connected to fault detection circuitry. In the absence of a fault or hazard on the lines of the system the current flowing toward the load is equal to the current returning back to the source. The equal currents in the primary windings of the transformer induce equal and opposing magnetic flux fields in the transformer core and no signal is coupled to the secondary winding. When a fault occurs in the system, such as a human becoming inadvertently connected from a current carrying line to ground, currents in the primary windings of the transformer no longer are equal and a resultant flux is established in the transformer core. This flux induces a signal in the secondary winding which initiates operation of a circuit breaker to open the lines of the system.
Ground Fault Current Interrupters of the type described above are effective to provide protection against many but not all types of faults that may occur. One type of fault that may not be detected by the described type of GFCI is one in which the neutral line of the supply system is intentionally or inadvertently grounded on the load side of the GFCI. The intentional grounding may occur because an electrician, appliance installer, or home owner violated an electrical code, most of which prohibit such a ground connection. In this situation, if grounded human comes into contact with the "hot" line of the supply system on the load side of the GFCI, current will flow through his body to ground. However, this current will not necessarily flow exclusively through ground to the ground connection on the neutral line that is at the source of current, it being assumed that the neutral conductor always is grounded at the source as is standard practice. It may happen that a portion of the current that flows through the human to ground will flow to the second improper or inadvertent ground on the load side and return to the source through the neutral conductor which is one primary winding of the GFCI transformer. In this situation the unbalance of currents in the primary windings of the GFCI may not be of sufficient magnitude to activate the fault detection circuitry of the GFCI. Consequently, the human may suffer injurious or fatal shock.